Approximately eighty percent of the refracting power of the eye is at the cornea. When the cornea is misshapen or the axial length of the eye is too long or too short, or when the lens of the eye is functioning abnormally, the refractive errors of myopia, astigmatism or hyperopia can result. Spectacles correct refractive errors by refracting the light with a lens before it reaches the cornea in order to change the angle at which light enters the cornea. Contact lenses correct refractive errors of the eye by replacing the misshapen cornea with the front curve of a contact lens which is calculated to render the eye emmetropic (a state where no visual correction is necessary). When the lens is taken off, however, the cornea is still misshapen or defective and refractive errors still exist.
The cornea itself is composed of five layers. The outermost layer is the epithelium, which is 4-5 cells thick. Beneath the epithelium is the acellular Bowmans membrane. The middle layer is the stroma, which is composed of scattered corneal fibroblasts (keratocytes) among organized lamellae of collagen, proteoglycans, and glycoproteins. Below the stroma is another acellular layer called Descemet's membrane. The innermost layer of the cornea, comprised of a single layer of flattened cells, is the endothelium.
The structural components of the human cornea are chiefly proteoglycans and collagens. Proteoglycans are composed of a hyaluronate core, a protein core, and glycosaminoglycans, which are proteoglycan monomers with repeating disaccharide units. Approximately 60% of the glycosaminoglycans of the cornea are made up of keratan sulfate, while the remaining 40% are mostly chondroitin sulfate. The other main structural component of the cornea, collagen, is found in seven different forms in the human cornea.
These structural components of the cornea are somewhat pliable and allow the cornea to be reshaped with a series of progressive contact lens changes to correct refractive errors. This procedure is known as Orthokeratology. The methods of Orthokeratology without the use of enzymes or other agents originated in 1962 as an extension of normal contact lens use. Orthokeratology is generally defined as the therapeutic use of contact lenses to reshape the corneal curvature, thereby improving refractive errors of the eye. Dr. Charles May and Dr. Stuart Grant are credited with pioneering the process. University and clinical level research over the next 20 years has confirmed the safety, effectiveness, and retention of this procedure. Orthokeratology has today become a contact lens specialty practice for a limited number of private practitioners, primarily in the United States.
Traditional Orthokeratology procedures use a series of progressive contact lens changes to reshape the cornea, thereby producing a cornea with a more spherical shape. This reduces or eliminates myopia and astigmatism and improves natural vision. Once a desired corneal curvature has been produced, retainer contact lenses are worn to stabilize the results. The contact lenses are made of a rigid gas permeable material and contain no enzymes or agents. The length of a program of treatment varies from six to eighteen months with progressive contact lens changes and examinations every two to six weeks.
A common fitting formula for Orthokeratology is as follows:
Lens Base Curve In Diopters=Flattest central corneal curvature in diopters to 1.0 diopter flatter.
Lens Diameter=Base curve in mm+1.5 mm ##EQU1##
Thickness=0.18 mm for 0 power-subtract 0.01 mm for each 1 diopter minus; add 0.02 mm for each 1 diopter of plus
Intermediate Curve=Base Curve in mm+1.5 mm, width=0.35-0.5 mm
Peripheral Curve=Base Curve in mm+3.0 mm, width=0.35-0.5 mm
When treating myopia, new contact lenses are refit with flatter curvatures, less correction, larger diameters, and greater thickness as the Orthokeratology program progresses. The patient's central corneal curvature continues to lessen (flatten and become more spherical), myopia and astigmatism are reduced, and unaided (natural) visual acuity improves significantly. When the maximum desired results are achieved or the patient ceases to improve, retainer contact lenses are worn full time or part time to retain the results.
For traditional Orthokeratology procedures, university and clinical research indicates the following limits of change: 4 diopters of myopia and 2.5 diopters of astigmatism change, no appreciable hyperopia change, 2 diopters of central corneal change, and 9 lines of unaided visual acuity change on the Snellen chart. Regression may occur in hours or days if retainer lenses are not worn.
Notwithstanding the foregoing, there remains a need for an improved method of correcting refractive errors in the eye nonsurgically which can correct larger degrees of refractive error and produce relatively permanent results in a much shorter period of time.